SUMMARY Breast cancer affects approximately 12% of all women in western and industrialized nations, resulting in a total number of 230,000 newly diagnosed cases each year in the United States alone. Although the majority of these women is cured of the disease, there still is a considerable annual mortality rate of 40,000, which strongly motivates the development of more effective treatment options. An extensive list of pharmacologically active drugs, known as chemotherapy, either given orally, systemically or locally, is available to treat breast cancer. Unfortunately, a number of intermediate- and long-term side effects are associated with systemic chemotherapy. A way to overcome chemotherapy's severe side effects is through more efficient delivery of the drug to cancerous lesions. This can be accomplished by nanoparticles, tiny carrier vehicles that can be loaded with drugs, known as nanomedicines. Doxil, a liposomal formulation of doxorubicin, was the first nanoparticle drug formulation to be approved for clinical use. Since its introduction in 1995, the nanomedicine field has undergone exceptional growth. The ability to non-invasively evaluate nanomedicine tumor targeting would greatly improve patient care by allowing swift adjustments in dosage and/or treatment regimen. In this multi-PI application, which will build on an ongoing collaboration between Mulder's nanomedicine group at Mount Sinai and Reiner's radiochemistry group at Memorial Sloan Kettering Cancer Center, we propose positron emission tomography (PET) nanoreporter technology that can be applied for clinical grade nanotherapeutics without the need for their chemical modification. Based on strong preliminary data, in this application we propose to 1) further advance our nanoreporter technology to clinically relevant settings, 2) use it to monitor tumor penetration enhancing therapies, and 3) generalize the nanoreporter concept to new chemotherapy nanomedicines. Our central hypothesis is that our PET nanoreporter technology allows outcome prediction in mouse models of cancer and can be adapted to a variety of clinically relevant anti-cancer nanomedicines. Through extensive validation imaging experiments and imaging guided therapeutic studies we will test our hypothesis in different mouse models of breast cancer and different disease contexts. We will also complement the non-invasive imaging data with flow cytometric cell targeting analysis for the different nanoformulations. Successful completion of the proposed science will provide valuable information for clinical translation of the nanoreporter technology in the post-award period.